Novel methods of transduction for active control of harmonic sound radiated by vibrating surfaces

摘要

Large electric transformers such as those used in high voltage substations radiate an annoyinglow frequency hum into nearby communities. Attempts have been made to actively control thenoise by placing a large number of loudspeakers as control sources around noisy transformersto cancel the hum. These cancellation systems require a large number of loudspeakers to besuccessful due to the imposing size of the transformer structures. Thus such systems are veryexpensive if global noise reduction is to be achieved.The aim of this thesis is to investigate theoretically and experimentally the use of thin perforatedpanels closely placed to a heavy structure to reduce the radiation of unwantedharmonic noise. These panels can themselves be vibrated to form a control source radiating overa large surface surrounding the primary source. The problem of the equipment overheating insidethe enclosure is alleviated because the holes in the panels still allow natural cooling.An initial study is carried out to determine the resonance frequencies of perforated panels. Theuse of previously determined effective elastic properties of the panels and Finite ElementAnalysis to theoretically calculate their resonance frequencies is examined.Secondly the attenuation provided by active noise control using perforated panels as controlsources is explored by use of a coupled analysis, where the primary source is assumed toinfluence the radiation of the perforated control panel. This analysis was found to predict poorlythe amount of attenuation that could be achieved, so an uncoupled analysis is undertaken, whereboth the primary and control sources are assumed to radiate independently of each other. Notonly does this greatly simplify the theoretical analysis but it also enables prediction of attenuationlevels which are comparable to those determined experimentally. The theoretical model isreformulated to enable comparison of the sound power attenuation provided by perforated panelcontrol sources with that of traditional acoustic and structural control sources.Finally, the use of modal filtering of traditional acoustic error sensor signals to give transformedmode (or power mode) sensors is examined. The independently radiating acoustic transformedmodes of the panel are determined by an eigenanalysis and a theoretical analysis is presented fora farfield acoustic power sensor system to provide a direct measurement of the total radiatedacoustic power. The frequency dependence of the sensor system, and the amount of global soundpower attenuation that can be achieved is examined. Experimental measurements are made toverify the theoretical model and show that a sound power sensor implemented with acousticsensors can be used in a practical active noise control system to increase the amount ofattenuation that can be achieved. Alternatively the sound power sensor can be used to reduce thenumber of error channels required by a control system to obtain a given level of attenuation whencompared to traditional error criteria. The power mode sensor analysis is then applied to theperforated panel control system, with similar results.